The intensities and frequencies of vibrational transitions measured in Raman spectra provide unique chemical signatures of molecular species, but the sensitivity of Raman spectroscopy suffers from the relatively low cross-section of inelastically scattered Raman photons. It has been demonstrated that the magnitude of Raman cross-sections can be greatly enhanced when the Raman-active molecules are placed on or near a roughened noble metal surface. Since then a wide variety of substrates have been found to enable surface enhanced Raman spectroscopy (SERS) such as aggregated noble metal colloids, metal island films, metal film over nanospheres, particles grafted on silanized glasses, regular holes in thin noble metal films and regular nanoparticle arrays.
In general, traditional SERS substrates can be divided into two fundamental substrate classes: random and engineered substrates. Random substrates like fractal nanoparticle agglomerates can support localized dipole modes which lead to high SERS signal enhancements. However, the resonance wavelength, the precise locations of the spots of giant |E|-field enhancement—so called hot-spots—and the reproducibility of their enhancement factors are difficult to control in completely random structures. Another disadvantage specific to fractal nanoparticle aggregates is that their mass density, and therefore the hot-spot density, decreases with increasing fractal size.
Challenging applications of SERS in single molecule spectroscopy or whole organism fingerprinting would greatly benefit from engineered SERS substrates with rational design criteria that generate high SERS enhancement reproducibly at spatially defined locations. Consequently, regular nanoparticle arrays and other nanofabricated SERS substrates, whose characteristic structural parameters can be accurately controlled, are attracting interest as SERS substrates with reproducible, high enhancement factors “by design”. The SERS enhancement in noble metal nanoparticle arrays depends on both the properties of the constitutive building blocks (nanoparticles) as well as the characteristics of their arrangement. In general two separate electromagnetic regimes govern the collective response of periodic metal-nanoparticle arrays: near and far-field coupling. When the particles are separated by short distances up to approximately D=1/k0=λ0/2π (with k0 and λ0 being the free space wavenumber and wavelength, respectively), strong quasi-static near-field interactions dominate the response of the array. Consequently, localized modes with strongly enhanced local fields are excited. When the particles are separated by larger distances, far-field diffractive coupling between the particles becomes dominant.
In the near-field coupling regime the field enhancement and corresponding SERS intensity arising from periodic arrays of nanoparticles sharply increases with decreasing inter-particle separation. Both theoretical and experimental studies have shown that regions of high |E|-field enhancement are located in the junction between individual particles. The |E|-field enhancement in these spatially confined hot spots can be orders of magnitude larger than on the surface of individual particles. Due to the rapid decay of the field strength with inter-particle separation and the |E|4 scaling of the SERS signal, very short inter-particle separations are vital in order to maximize the Raman enhancement in the near-field coupling regime. Ideally, the analyte molecules are placed in the junctions between nearly touching metal surfaces.